The atomic mass of lithium is low and its standard electrode potential is negative. In the light of these properties lithium is latently well suited for making electrochemical cells (secondary battery elements) of high energy density. When used with a suitable positive electrode, high terminal potentials of more than three volts are possible. However, high terminal potentials entail the requirement that the electrolyte solution covering the positive and negative electrodes have a high decomposition voltage. If not, consumption of the active material of one of the electrodes and/or of the electrolyte solution occurs, and accordingly no useful battery element can be achieved. The active material, as used herein and in the art, is that component of an electrode which is converted during the electrode reaction and thereby determines the electrochemical properties of the cell.
In recent years a number of non-rechargeable electrochemical primary batteries that are lithium based have been developed, which exhibit high energy densities.
As regards the rechargeable electrochemical secondary batteries, they must meet further requirements in addition to high energy density. In particular such a cell should be rechargeable over a high number of cycles, while the capacity should remain as constant as possible, and also high currents should be possible both during the discharge and charge cycles.
It is known, for instance, from British patent documents A 2,083,942, and 2,142,469 and U.S. Pat. No. 4,891,281, to use an inorganic electrolyte solution wherein sulfur dioxide (SO.sub.2) is the solvent. Chloride-containing electrolytes, in particular salts of lithium with a halide of a 3A element, are used therein. Illustratively such salts are LiAlCl.sub.4 and LiGaCl.sub.4. These known rechargeable cells however are not fully satisfactory as regards the possible number of charging/recharging cycles and long-term operability.
The object of an invention is to provide a rechargeable electrochemical cell which is characterized by a high energy density, a high number of possible charging and discharging cycles, high currents and good long-term operability.